This application claims the priority of German Patent Application No. 198 05 928.0, filed Feb. 13, 1998, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a method for determining the fullness (i.e., remaining capacity) or quality of a catalytic converter that stores gases using a storage medium.
Increasingly, strict air pollution laws combined with the pressure to reduce fuel consumption require new concepts for both internal combustion engines and exhaust scrubbing. This also means new concepts for monitoring exhaust scrubbing systems.
The problems described below occur particularly in conjunction with catalytic converters that store gases.
In a stoichiometrically operated four-cycle engine (so-called ".lambda.=1 engine") the air/fuel ratio .lambda. of the raw exhaust is detected using a first .lambda. probe. Depending on the deviation from the ideal state (.lambda.=1), the air/fuel ratio is then adjusted slightly. In practice, this produces a .lambda.-oscillation of the raw exhaust around the stoichiometric point (.lambda.=1). However, .lambda.=1 must be maintained in a time average. See Wiedenmann et al., "ZrO.sub.2 Lambda Probes for Regulating Mixtures in Motor Vehicles," in Schaumburg, Hanno, Sensor Applications, Teubner Verlag, Stuttgart (1995), 371-399.
The oscillation frequency lies in the range of several tenths of a second to several seconds. Because of the ability of a so-called "three-way catalytic converter" located downstream from the first .lambda. probe to store oxygen, optimum conversion always takes place provided the catalytic converter is still in good condition. As the quality of the catalytic converter deteriorates (i.e., the conversion rate decreases and the response temperature increases), the ability of the catalytic converter to store oxygen likewise decreases. A second .lambda. probe located downstream from the catalytic converter will then be able to detect the oscillation of the air/fuel ratio. The quality of the catalytic converter can be determined by evaluating the ratio between the amplitudes of the first and second .lambda. probes. Glokler & Mezger, Self-Diagnosis of Modern Engine Control Systems, "State of Development and Initial Experiences with OBD2 for the USA," 38-52. In this method, only an indirect method is involved, which encounters limits in novel high-efficiency catalytic converters like those used for ULEV requirements, for example.
Many oxides of nitrogen are produced in an internal combustion engine operated with an air surplus (the so-called "lean engine" or Diesel engine, for example). One possible exhaust scrubbing concept provides for installing in the exhaust line of a motor vehicle a catalytic converter that can store nitrogen oxides for a certain period of time. Following this "storage phase" in which the catalytic converter is "filled" with the exhaust component to be stored, a desorption phase follows in which the catalytic converter is "drained". See Strehlau et al., "New developments in lean NOx catalysis for gasoline-fueled passenger cars in Europe," SAE Paper 962047 (1996).
In current concepts for determining the fullness of the catalytic converter and the regulation of the fuel/air ratio associated therewith, gas sensors are used which measure the gas to be stored (NOx), downstream from the catalytic converter. A breakthrough by the gas downstream from the catalytic converter indicates that the catalytic converter is filled with the gas to be stored and that "drainage" (the so-called desorption phase) must be initiated. Patents and publications about NOx sensors exist in great numbers. Such patents include, for example, European patent document EP 0 257 842; U.S. Pat. No. 5,466,350; and German patent document DE 4308767. Such publications include, for example, (1) Rao et al., "Potentiometric NOx (x=1,2) Sensors with Ag-.beta." as Solid Electrolytes and Ag Metal as Solid Reference," Solid State Ionics 52 (1992) 339-346; (2) Kato et al., "Thick Film ZrO.sub.2 NOx sensor," SAE Paper 960334 (1996); and (3) Somov et al., "Gas Analysis with Arrays of Solid-State Electrochemical Sensors: Implications to Monitor HCs and NOx in Exhausts," Sensors and Actuators, B. 35-36 (1996) 409-418. However, such sensors do not actually- detect the fullness of the catalytic converter, but rather the NOx content of the exhaust. In addition, many sensors have stability problems and are cross-sensitive to oxygen and/or water and carbon monoxide in addition to NOx. Another disadvantage of this method is that a signal to the engine control to "clear out" (i.e., drain or desorb) can only be given after the breakthrough of the gas from the catalytic converter has already occurred.
The catalytic converter temperature is insufficient to catalytically burn the hydrocarbons that are produced during a cold start. In addition to proposed solutions that provide electrical preheating (Otto et al., "System development of the electrically heatable catalytic converter for the LEV/ULEV and EU3 legislation, " MTZ 56 (9) (1995) 488 et seq.) or which provide for a firewall catalytic converter to be located very close to the engine in combination with secondary air being blown in (Albrecht et al., "BMW 6-Cylinder Technology for TLEV and OBD2 Requirements in the USA," MTZ 57 (10) (1996)), systems that store hydrocarbons have also been discussed which give up their stored hydrocarbons when the starting temperature is reached. Hydrocarbon sensors are required to monitor such systems but they also have problems with stability and cross-sensitivity.
The object of the present invention is to provide a method for determining the fullness or the quality of catalytic converters that store gases, which overcomes the disadvantages of the prior art. Such catalytic converters can be used for scrubbing exhaust gases of motor vehicles.
According to the present invention, this object is achieved by direct methods for detecting the quality of a catalytic converter or for detecting the fullness of a catalytic converter. These methods detect the conversion of a catalytic converter coating (also referred to as a storage medium because of its storage ability) on the basis of a chemical interaction between the gas to be stored (e.g., oxygen, carbon monoxide, nitrogen oxides, or hydrocarbons) and the coating. Thus, the quality of the catalytic converter and/or the fullness of the catalytic converter is detected.
In the chemical interactions that take place during storage or draining, the chemical state of the catalytic converter coating of the monolith changes. The coating is typically composed of wash coat, noble metals, and storage elements. In a three-way catalytic converter, as the conversion rate declines and the quality of the catalytic converter decreases, the nature and the structure of the active catalytic surface changes as well. As the coating changes, the physical properties also change, for example, the complex dielectric constant, the electrical conductivity, index of refraction, and the like.
The fullness of the catalytic converter and the diagnosis of the catalytic converter quality can be performed by determining those physical properties of the catalytic converter coating that change with the storage process. In particular, the fullness or the quality of the catalytic converter can be determined by measuring the complex electrical impedance Z, which also includes the electrical DC resistance. The complex electrical impedance Z is defined as the sum of the real part Re[Z] and the imaginary part Im[Z] of the complex impedance Z. The electrical impedance Z changes with the applied measuring frequency. A suitable frequency range is greater than or equal to 0 Hz (DC). An upper limiting frequency may be at a wavelength corresponding to a measurement frequency that is much smaller than the dimensions of the measuring arrangement. Preferably, a suitable measuring frequency is chosen and the complex impedance is determined at this frequency from the real part and the imaginary part, or a measurement signal derived from one or both of these two values is recorded. Typical measurements include the following: the value of the complex impedance .vertline.Z.vertline., the DC resistance, the capacitance, the loss factor, or the detuning of the oscillating circuit. Additional electrical measured values that can be used are the Seebeck coefficient (also called the thermal electromotive force) or the temperature curve of the abovementioned electrical values.
Additional non-electric measured values that can be used to determine the fullness or quality of the catalytic converter are the optical index of refraction, the optical absorption or reflection coefficient, or the change in mass or volume.
Instead of measuring the physical properties of the storage medium itself, a medium that is identical or similar to the storage medium of the catalytic converter as far as its physical properties are concerned can be located separately from the catalytic converter in the exhaust stream and the changes in the physical properties of this identical or similar medium can be measured.